1. Field of the Invention
The present invention relates to a key groove machining pass generating apparatus for a wire electric discharge machine.
2. Description of the Related Art
Japanese Patent Laid-Open No. 8-153132 discloses a CAD/CAM apparatus with which an operator can automatically draw a figure, generate data of key groove machining, and generate NC data simply by selecting defined holes and inputting parameters concerning a key and a key groove shape.
The apparatus disclosed in Japanese Patent Laid-Open No. 8-153132 can generate NC data for key groove machining by including a key groove defining unit and a key groove database registering unit. However, the apparatus is configured to generate the NC data without taking into account a problem that occurs when wire electric discharge machining is applied to an arcuate surface to be machined. Therefore, a key sometimes does not enter a key groove.
A problem that occurs when a workpiece is cut in wire electric discharge machining is explained with reference to FIGS. 10 and 11. FIG. 10 is a diagram showing a state in which a wire electrode 2 receives a force in a direction opposite to a wire traveling direction 6 if a surface to be machined 4a and a machining pass 8 are orthogonal to each other when a workpiece 4 is cut in the wire electric discharge machining. FIG. 11 is a diagram showing a state in which the wire electrode 2 deviates from the machining pass 8 if the surface to be machined 4a and the machining pass 8 are not orthogonal to each other when the workpiece 4 is cut in the wire electric discharge machining. Note that reference numeral 10 denotes electric discharge.
As shown in FIG. 10, when the wire electrode 2 is cut into the workpiece 4 in the wire electric discharge machining, if the surface to be machined 4a and the machining pass 8 are orthogonal to each other, a discharge repulsive force generated in the wire electrode 2 and a force 12 received from machining liquid by the wire electrode 2 work in a direction opposite to a traveling direction of the wire electrode 2 along the machining pass 8.
However, as shown in FIG. 11, if the surface to be machined 4a and the machining pass 8 are not orthogonal to each other, the two forces 12 work to move away from the machining pass 8 according to an angle formed by the surface to be machined 4a and the machining pass 8. The wire electrode 2 deviates from the machining pass 8 with the forces, which is problematic. Consequently, this causes a problem that when a key groove is machined, shape accuracy of an opening of the key groove is deteriorated, the dimensions of the opening decrease and a key cannot be inserted into the key groove.
FIG. 12 is a diagram for explaining a conventional machining method for machining a key groove in a round hole. When a key groove 16 is machined in a round hole 15 shown in FIG. 12, usually, a machining pass 18 indicated by a dotted line having a shortest machining distance is generated to perform the machining. When the machining is applied to a surface to be machined 14a of the round hole 15 on such a machining pass 18, since the surface to be machined 14a and the machining pass 18 are not orthogonal to each other, the forces explained above work on the wire electrode 2. The wire electrode 2 deviates from the machining pass 18 as indicated by an actual moving pass 20 of the wire electrode 2 shown in FIG. 13. As a result, a cutting portion of a key groove is formed in a machining shape like a part 24 shown in FIG. 14. A key groove opening is narrower than the width indicated in the drawings and the like. Therefore, a key 22 shown in FIG. 14 does not enter the machined key groove.